Method of fracturing a subsurface formation



Oct. 15, 1963 J. L. HUlTT ETAL METHOD OF FRACTURING A SUBSURFACE FORMATION Filed April 15, 1960 INVENTORS J/MM/E .4. H0177 eke/c5 a. MC azoawu/v BYJOJEPH A. PE/(AEEK United States Patent Oflice 3,106,959 Patented Oct. 15, 1963 I V r 3,106,959 METHOD OF FRACTURING A SUBSURFACE FORMATION Jimmie L. Huitt, Glenshaw, Bruce B. MeGlothlin, Pittsburgh, and Joseph L. Pekarek, Penn Hills, Pa., assignors to Gulf Research 8: Development Company, Pittsburgh, Pa., a corporation of Delaware Filed Apr. 15, 1960, Ser. No. 22,494 11 Claims. (Cl. 166-37) This invention relates to the treatment of wells such as oil wells and the like, and more particularly to methods for positioning and orienting artificial fractures in subterranean formations.

It is well known in the art of well stimulation to inject a fluid into an earth formation and to apply pressure to the fluid to create artificial fractures in the formation. The creation of artificial fractures in fluid-bearing earth formations facilitates drainage of such formations into a well from which the desired fluids can be recovered. In hydraulic fracturing operations a fracturing liquid, generally a viscous or non-penetrating liquid, is forced down the well and into the formation under sufficient hydraulic pressure to overcome the tensile strength of the formation, that is, to break down the formation thereby causing a crack to be produced in the formation. The fracturing fluid upon being forced into the induced crack extends the fracture substantial distances into the formation. Usually a propping agent such as sand is suspended in the fracturing fluid for deposit in the fracture to prevent closure of the newly created drainage channels by the pressure of the overburden of earth.

Although hydraulic fracturing treatments have come into widespread use in the art of well stimulation, the fracturing process is not always successful in increasing the production of the well. The principal cause of unsuccessful fracturing operations is probably the lack of control of the direction and orientation of the fracture. Under the pressure of the hydraulic fluid the fracture is ordinarily initiated and extended along a natural plane of weakness in the formation which, of course, may extend in any direction. As a result the fractures often extend out of the oil-bearing rock or out of the desired interval to be fractured into a gas cap above the oil in the pay zone or into water below the oil. The low viscosity of the gas or the water as compared with the oil may result in the gas or water flowing readily through the fracture to the well leaving the oil in the formation.

In attempts to control the location at which a fracture is created, several techniques have been developed. For example, a formation or casing packer either single or dual can be set in the well to isolate and confine a selected producing Zone in which a fracture is desired. This technique provides control over the general location of the fracture but gives no control over the direction or orientation of the fracture.

The present invention provides a method for accurately positioning and orienting fractures in subterranean formations. The process of the invention is carried out by positioning within a well a conventional metal pipe string such as a casing or liner having installed therein one or more sections which project outwardly beyond the circumference of the casing and which when the casing is set will be positioned opposite the producing formation. The outwardly projecting sections of casing, or casing inserts, taper to a reduced thickness at the outer or leading edge thereof and are composed of a material which permits removal of the section of easing while leaving the remainder of the casing intact within the well. The metal pipe string including the removable outwardly projecting sections thereof is cemented in place in the well in conventional manner, the removable casing inserts functioning in all respects as an integral part of the casing. Thereafter, the removable outwardly projecting sections of easing are removed in any suitable manner thereby exposing to the bore of the casing the cement sheath in which there is located a notch corresponding in size and shape to the outwardly projecting casing section. Hydraulic pressure is then applied to the notch in the cement sheath to fracture the formation. Upon application of hydraulic pressure, large stress concentrations are set up at the apex of the notch in the cement sheath and are transmitted to the formation to cause the formation to fracture in substantially the plane of the notch.

In cementing the metal casing within the well, it is of utmost importance that a strong bond be produced between the walls of the formation and the removable sections of easing. A good cement job not only insures that the fracturing pressures are transmitted to the formation wall in the desired plane in the potentially producing strata, but also isolates the fracture from other well production. The casing may :be moved during placement of the cement sheath to insure that the cement intimately surrounds the removable casing sections and abrading means can also be employed to remove the mud sheath and to produce a strong bond between the well walls and the metal casing.

The outwardly projecting sections of easing which in accordance with this invention are removed after the casing is set in the well can be composed of a variety of materials which permit removal of the desired casing sections while leaving the remainder of the casing intact. Thus, for example, the removable casing inserts can be composed of metals such as aluminum which metal is soluble in acids or strong alkalis, or magnesium metal which is readily soluble in acids. Acidor alkali-soluble resins can also be employed. In addition, the removable casing inserts can be composed of materials which melt at relatively low temperatures and which can therefore be removed by applying heat thereto within the well bore. Thus, for example, the removable casing inserts can be composed of metals such as lead or tin or alloys comprising mixtures of lead-tin, lead-tin-bisrnuth-cadmium and like materials which in general melt at temperatures in the range from about 200 to 700 F.

A suitable aluminum alloy for use in fabricating the removable casing sections is one containing about 1 percent silicon, about 1 percent magnesium and about 0.3 percent chromium, the remainder being aluminum. A suitable magnesium alloy is one containing about 3.5 percent aluminum, about 0.2 percent manganese and about 0.3 percent zinc, about 0.5 percent silicon, traces of copper and nickel and the remainder being magnesium. As between the magnesium alloys and the aluminum a1- loys, the former are preferred for use in accordance with the present invention for several reasons. First of all, they can be welded to steel and are lighter for the same strength. Secondly, they are much more rapidly dissolved in acid which contains inhibitors to protect the steel casing and tubing. Sections of easing composed of aluminum alloys can readily be removed by the chemical action of strong alkalis such as sodium hydroxide. When the removable casing sections are composed of magnesium alloys of the preferred type, the dissolving agent can be hydrochloric acid having a strength in excess of about 5 percent and preferably between 15 and 30 percent. This acid will preferably contain inhibitors of the type known to the art to protect the steel tubing from corrosion. The acid is placed in contact with the removable casing sections by circulating the acid down a pipe which has an open lower end positioned below the removable casing section. Another method of introducing the acid is by means of a closed container that is lowered on a wire line to a point opposite the removable casing section and from which the acid is discharged by a suitable mechanism at the desired time.

The removable casing sections project outwardly beyond the circumference of the remainder of the casing a sufficient distance to produce a well-defined notch in the cement sheath surrounding the section of casing. The removable casing sections taper to a reduced thickness at the outer or penetrating edge and preferably taper to a relatively sharp point at the outer or penetrating edge so as to form a V-shaped notch in the cement sheath. While the removable casing sections extend outwardly beyond the normal circumference of the casing, sufficient clearance should be provided between the outer edge of the casing insert and the borehole wall to permit circulation of the cement down through the annulus between the casing and the formation wall. This clearancealso facilitates maneuvering the easing into the borehole. A suitable casing insert for use in a 8% inch diameter borehole cased with /2 inch casing, is one which projects about one inch beyond the circumference of the remainder of the casing and is tapered to define an angle of about 10 degrees to 45 degrees at its outer edge.

The radial direction of the fractures produced in accordance with the invention is determined by orientation of the casing within the well and more particularly by the orientation of the removable casing sections. The casing can be constructed with one or several radially disposed removable sections for the purpose of creating a fracture in the formation in given radial directions as is desired, for example, is establishing fluid communicaton between two wells for in-situ combustion processes and the like. However, when it is desired to create in a producing formation a horizontal fracture radiating in all directions 360 around the borehole, the casing is constructed with a readily removable section which extends around the entire circumference of the casing and which upon removal from the casing exposes an annular notch in the surrounding cement sheath.

Not only is the compass direction of the fracture positively controlled by orientation of the removable casing section within the well, but also the fracture is oriented as to the angle the fracture plane makes with the horizon. Thus, the outwardly projecting removable casing sections can be so positioned within the well bore to create horizontal, or vertical fractures. For horizontal fractures the outwardly projecting casing inserts are positioned. so that the outer penetrating edges thereof lie in a horizontal plane. If a vertical fracture is desired, the casing inserts are positioned such that the outer penetrating edges thereof are disposed in a vertical plane in the compass direction in which the fracture is to extend.

After the removable casing section is removed from the well thereby leaving a notch in the cement sheath, a hydraulic pressure is applied thereto to fracture the earth formation.

"Application of the hydraulic pressure causes a fracture to extend from the apex of the notch into the formation in substantially the same plane as the notch. The hydraulic pressure is provided by injecting into the well any of the fracturing fluids commonly employed in conventional hydraulic fracturing operations. These fluids for the most part, comprise hydrocarbon gels obtained by dispersing a suitable gelling agent in a hydrocarbon oil such as gasoline, kerosene or crude oil. Commonly employed gelling agents include aluminum naphthenate and mixtures thereof, with aluminum oleate, hydroxy aluminum soaps of fatty acids, aluminum and other metal soaps of the various fatty acids derived from coconut oil, peanut oil and the like. Aqueous fracturing fluids which may or may not contain a thickening agent such as a natural gum, as for example, Karaya, Batu or Guar gum, which will allow the build-up of pressures suflicient to extend the fracture in the formation can also be employed.

The hydraulic fluid employed to extend the fracture in accordance with the invention may contain a granular solid propping agent. Thus, the hydraulic fluid may contain as props, materials such as glass beads, ceramic particles, metal particles, crushed rock, wood chips, and the like. Sand because of its cheapness and general availability is a preferred propping material. The particle size of the propping material can range from those quite small enough to pass through a #4 standard mesh sieve to those just large enough to be retained on a mesh sieve.

Further details of the invention will become apparent from the following detailed description when read in conjunction with the accompanying drawings which illustrate several useful embodiments in accordance with this invention.

In the drawings:

FIGURE 1 is a cross-sectional longitudinal view of one embodiment of the invention showing a pipe string having a removable circumferential section installed therein positioned in a well bore opposite a producing formation.

FIGURE 2 is a view similar to FIGURE 1 showing the notch produced in the cement sheath after removal of the removable casing section.

FIGURE 3 is a longitudinal cross-sectional view of a pipe string in accordance with another embodiment of the invention.

FIGURE 4 is a cross-sectional view along line IVIV of FIGURE 3.

Referring to FIGURE 1, there is shown a well bore 10 which has been drilled by conventional methods and means through an earth formation '11 which contains a fluid such as oil or gas which is sought to be produced. The formation 11 is positioned between overlying and underlying earth formations 2 and respectively, one or both of which contain undesired connate fluids such as water. The apparatus for completing the well for production includes a pipe string designated generally by the numeral 14 which is somewhat smaller in diameter than the well "bore 10 and which is inserted into the well bore from the surface and extends therefrom through earth formation 11 in accordance with conventional practice.

Included in the pipe string 14 is a circumferential removable section 15 constructed of magnesium alloy as heretofore described. As seen, removable section 15 extends around and projects outwardly a substantial distance beyond the circumference of the pipe string 14 and is tapered to a relatively sharp point at its outer edge 16. Tubing 17 which can be a macaroni string or other small size tubing is provided for introduction of chemical reagents for reaction with the magnesium comprising section 15.

In carrying out the method of the present invention with the above-described apparatus, the pipe string 14 having the circumferential annular outwardly projecting section 15 installed therein is lowered into well bore 10 to position section 15 opposite earth formation 11. Pipe string 14 is then cemented in place by employing conven tional cementing procedures. As is well understood by those skilled in the art, a cement slurry is pumped into the bore of the pipe string 14 at the surface and is displaced downwardly therethrough by means of a suitable displacing fluid which forces the cement out of suitable openings in the pipe string at some position along formation 13 below formation 11 and thence up through the annular space between the pipe string and wall of the borehole 10 to a level above removable section 15. Portland or other hydraulic cement may be used to produce the cement sheath as well as plastics, or like material conventionally used in setting casing. When the cement indicated at 18 hardens it forms a seal between the pipe string 14 and the well bore 10 to thereby seal-off formation 11 from the overand underlying earth formations 12 and 13 respectively.

After the pipe string 14 has been cemented in place and the cement has set, hydrochloric acid of about 20 percent strength is pumped into the well through tubing 17. Introduction of the hydrochloric acid is continued until section of the pipe string 14 is completely corroded away. With a constant feed of acid into the section the completion of the chemical reaction can be determined by a decreased rate of flow of the spent acid. Another method to determine when the job is completed is to analyze the spent acid for magnesium. As a practical matter, experience will indicate how much time is necessary for sufficient removal of section 15 so that the operation can be conducted on a time basis without the necessity of resorting to chemical analysis.

The removal of the section 15 from the pipe string 14 exposes annular notch 19 in cement sheath 18. As is seen in FIGURE 2, the annular notch 19 in the cement sheath 18 is of V-shape and corresponds in size to the outwardly projecting removable section 15 of pipe string 14. After removal of the section 15 from pipe string 14 and exposure of the V-shaped notch 19 in the cement sheath, fluid pressure is then applied to the notch to fracture the formation. The fluid pressure is applied by pumping into the well under pressure a fracturing fluid such as a crude oil gelled with a sodium soap of a fatty acid. The fracturing fluid is continuously pumped into the well until the pressure which is built up therein is sufficient to rupture the cement sheath at the apex of the notch 19 and to overcome the overburden thereby fracturing the earth formation. This event is usually indicated by a sudden decrease in pump pressure. After the overburden pressure is overcome and the fracture occurs, a propping agent comprising quartz sand particles of about 8 to 10 mesh size is incorporated into the fracturing fluid. The propping agent is mixed into the fracturing fluid as it is pumped into the well at a rate to provide about 3 pounds of propping agent per gallon of fracturing fluid. The fracturing fluid con taining the propping agent in suspended form is cont-inuously pumped into the Well to extend the fracture and to deposit the propping agent within the fracture. After the well has been shut-in for a sufficient time, the viscosity of the fracturing fluid is sufiiciently reduced by commingling with the formation fluids to permit it to be withdrawn from the well as part of the well eflluent during subsequent production.

FIGURES 3 and 4 illustrate another form of apparatus embodiment suitable for use in accordance with the invention. In this apparatus embodiment, the metal pipe string 14 has installed therein two diametrically opposed longi tudinally-extending removable inserts 20 and 21 for the purpose of creating vertical fractures in the formation 11 spaced 180 from one another. These sections are elongated in form having a length such as to extend throughout a substantial portion of the producing formation 11. The sections 20 and 21 are tapered so as to have relatively sharp penetrating edge surfaces as at 22 and 23 respectively, and are composed of a corrodible metal such as magnesium or aluminum or other non-metallic corrodible materials which permit the sections to be readily removed from the pipe string. This apparatus embodiment is employed in the same manner as the previously described embodiment. The pipe string 14 is installed in the well bore to position sections 20 and 21 opposite the earth formation 11. The pipe string 14- is then cemented in place in the usual manner. After setting of the cement the re movable pipe inserts 2t and 21 are removed by suitable means to expose the vertical notches 24 and 25 in the cernent sheath 18. Hydraulic pressure is then applied to the vertical notches 24- and 25 in the cement sheath '18 to produce vertical fractures in the earth formation 11.

From the foregoing it is seen that the invention provides a method for controlling positively the location and orientation of artificially induced fractures in subterranean formations. By means of the invention, fractures are easily produced in earth formations at predetermined depth and in predetermined planes.

It is to be understood that a variety of materials can be employed for constructing the removable sections of the pipe string. Thus, such materials as hard plastics, ceramic materials, cast metals such as cast magnesium or cast aluminum and the like, can be employed. In general, any material can be used which has substantial tensile and compressive strength but which will have properties which permit its removal by suitable means in preference to adjacent portions of the pipe string.

Those modifications and equivalents which fall within the spirit of the invention and the scope of the appended claims are to be considered part of the invention.

We claim:

1. A method of creating fractures at predetermined depth and orientation in an earth formation penetrated by a well bore which comprises: placing within said well bore a pipe string having therein WWI which projects outwardly beyond t e circumference of the pipe string and which is composed of a material cgnvertible to a liquid under conditions leavingdhe remainder of the WWW p1pe strlng substantially unaffected, sald removable p1pe section being'tap'red to a reduced ll icknessfiftthe outer projecting edge thereof, cementiiig the pipe string in place so that the said removable section thereof is opposite the earth formation in which a fracture is desired, converting the section of pipe to a liquid whereby said section of pipe is removed to expose a section of the cement sheath in which there is located a notch corresponding substantially in size and shape to the removable outwardly projecting pipe section, and then injecting a fluid into said Well bore under pressure suflicient to rupture the cement sheath and to fracture the formation at the location of the said notch.

2. A method of creating fractures at predetermined depth and orientation in an earth formation penetrated by a well bore which comprises: placing within said well bore a pipe string having therein a removable pipe section which projects outwardly beyond the circumference of the pipe string and which is composed of a material soluble in a chemical solution which will dissolve said section without detrimentally affecting the remainder of the pipe string, said removable pipe section being tapered to a reduced thickness at the outer projecting edge thereof, cementing the pipe string in place so that the said outwardly projecting pipe section is opposite the earth formation in which a fracture is desired, dissolving said outwardly projecting pipe section by means of a chemical which will dissolve said section without detrimentally affecting the remainder of the pipe string to thereby expose a section of the cement sheath in which there is located a notch corresponding substantially in size and shape to the outwardly projecting pipe section, and then injecting a fluid into said well bore under pressure suflicient to rupture the cement sheath and to fracture the formation at the location of the said notch.

3. A method according to claim 2 in which the said fracturing fluid contains a propping agent.

4. A method of creating fractures at predetermined depth and orientation in an earth formation penetrated by a well bore which comprises: placing within said well bore a pipe string having therein a removable pipe section which projects outwardly beyond the circumference of the pipe string and which is composed of a material having a melting point lower than the material comprising the remainder of the pipe string, said removable pipe section being tapered to a reduced thickness at the outer projecting edge thereof, cementing the pipe string in place so that the said outwardly projecting pipe section is opposite the earth formation in which a fracture is desired, melting said outwardly projecting pipe section to thereby expose a section of the cement sheath in which there is located a notch corresponding substantially in size and shape to the outwardly projecting pipe section, and then injecting a fluid into said well bore under pressure sufficient to rupture the cement sheath and to fracture the formation at the location of the said notch.

5. A method according to claim 4 in which the said fracturing fluid contains a propping agent.

6. A method of creating fractures at predetermined depth and orientation in an earth formation penetrated by a well bore which comprises: placing within said well bore a pipe string having therein a substantially horizontal circumferential removable pipe section which projects outwardly beyond the circumference of the pipe string and which is composed of a material soluble in a chemical solution which will dissolve said section without detrimentally affecting the remainder of the pipe string, said removable pipe section being tapered to a relatively sharp point at the outer projecting edge thereof, cementing the pipe string in place so that the said removable section is opposite the earth formation in which a fracture is desired, dissolving said removable pipe section by means of a chemical solution which will dissolve said section without detrimentally affecting the remainder of the pipe string to thereby expose a section of the cement sheath in which there is located a continuous circumferential V-shaped notch, and then injecting a fluid into said well bore under pressure sufiicient to rupture the cement sheath and to create a horizontal fracture in the earth formation at the location of the said notch.

7. A method of creating fractures at predetermined depth and orientation in an earth formation penetrated by a well bore which comprises: positioning within said well bore a pipe string having therein a plurality of elongated lon'gitudinally-extending removable pipe sections which project outwardly beyond the circumference of the pipe string and which are composed of a material soluble in a chemical solution which will dissolve said sections without detrimentally affecting the remainder of the pipe string, said removable pipe sections being tapered to a relatively sharp point at the outwardly projecting edges thereof, cementing the pipe string in place so that the said removable pipe sections are opposite the earth formation in which a fracture is desired, dissolving said removable pipe sections by means of a chemical solution which will dissolve said sections without detrimentally affecting the remainder of the pipe string to thereby expose sections of the cement sheath in which there are located in a plurality of vertically extending V-shaped notches, and then injecting a fluid into said well bore under pressure suflicient to rupture the cement sheath and to create vertical fractures in the earth formation at the location of the said notches.

8. A method of creating a fracture at a predetermined depth and orientation in a subsurface formation penetrated by a well bore which comprises cementing within said well bore casing having therein an insert extending through the wall of the casing of a material readily soluble in an acidic solution whereby the casing is surrounded by a sheath of cement, said insert having an outer surface tapering to a reduced thickness at its outer edge which projects outwardly beyond the circumference of the casing into the sheath of cement, displacing an acidic solution down the easing into contact with the insert to dissolve the insert thereby creating an opening in the casing and a notch in the cement sheath, and then injecting a fluid into said well bore under pressure to create a fracture in the earth formation at the location of the notch in the cement.

9. A method as set forth in claim 8 in which the casing is steel and the insert is magnesium.

10. A method of creating a fracture at a predetermined depth and orientation in a subsurface formation penetrated by a well bore which comprises cementing within said well bore casing having therein an insert extending through the wall of the casing of a material readily soluble in an alkaline solution to form a cement sheath surrounding the casing, said insert having an outer surface tapering to a reduced thickness at its outer edge which projects outwardly beyond the circumference of the casing into the cement sheath, displacing an alkaline solution down the casing into contact with the insert to dissolve the insert thereby creating an opening in the casing and a notch in the cement sheath surrounding the casing, and then injecting a fluid into said well bore under pressure to create a fracture in the earth formation at the location of the notch in the cement.

11. A method as set forth in claim 10 in which the easing is steel and the insert is aluminum.

References Cited in the tile of this patent UNITED STATES PATENTS 2,261,292 Salnikov Nov. 4, 1941 2,547,778 Reistle Apr. 3, 1951 2,642,142 Clark June 16, 1953 2,758,653 Desbrow Aug. 14, 1956 2,855,049 Zandmer Oct. 7, 1958 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 106,959 October 15, 1963 Jimmie L. Huitt et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

for "is" read in column 4, line same line 35, after "and insert strike out "in", second occurrence.

Column 3, line 31, 35, for "2" read 12 13 column 7, line 42,

Signed and sealed this 14th day of April 1964.

(SEAL) EDWARD J. BRENNER Attest:

ERNEST W. SWIDER Attesting Officer Commissioner of Patents 

1. A METHOD OF CREATING FRACTURES AT PREDETERMINED DEPTH AND ORIENTATION IN AN EARTH FORMATION PENETRATED BY A WELL BORE WHICH COMPRISES: PLACING WITHIN SAID WELL BORE A PIPE STRING HAVING THEREIN A REMOVABLE PIPE SECTION WHICH PROJECTS OUTWARDLY BEYOND THE CIRCUMFERENCE OF THE PIPE STRING AND WHICH IS COMPOSED OF A MATERIAL CONVERTIBLE TO A LIQUID UNDER CONDITIONS LEAVING THE REMAINDER OF THE PIPE STRING SUBSTANTIALLY UNAFFECTED, SAID REMOVABLE PIPE SECTION BEING TAPERED TO A REDUCED THICKNESS AT THE OUTER PROJECTING EDGE THEREOF, CEMENTING THE PIPE STRING IN PLACE SO THAT THE SAID REMOVABLE SECTION THEREOF IS OPPOSITE THE EARTH FORMATION IN WHICH A FRACTURE IS DESIRED, CONVERTING THE SECTION OF PIPE TO A LIQUID WHEREBY SAID SECTION OF PIPE IS REMOVED TO EXPOSE A SECTION OF THE CEMENT SHEATH IN WHICH THERE IS LOCATED A NOTCH CORRESPONDING SUBSTANTIALLY IN SIZE AND SHAPE TO THE REMOVABLE OUTWARDLY PROJECTING PIPE SECTION, AND THEN INJECTING A FLUID INTO SAID WELL BORE UNDER PRESSURE SUFFICIENT TO RUPTURE THE CEMENT SHEATH AND TO FRACTURE THE FORMATION AT THE LOCATION OF THE SAID NOTCH. 